This invention relates generally to plasma processing. More particularly, the invention is for plasma processing of devices using an inductive discharge. This invention is illustrated in an example with regard to plasma etching and resist stripping of semiconductor devices. The invention also is illustrated with regard to chemical vapor deposition (CVD) of semiconductor devices. But it will be recognized that the invention has a wider range of applicability. Merely by way of example, the invention also can be applied in other plasma etching applications, and deposition of materials such as silicon, silicon dioxide, silicon nitride, polysilicon, among others.
Plasma processing techniques can occur in a variety of semiconductor manufacturing processes. Examples of plasma processing techniques occur in chemical dry etching (CDE), ion-assisted etching (IAE), and plasma enhanced chemical vapor deposition (PECVD), including remote plasma deposition (RPCVD) and ion-assisted plasma enhanced chemical vapor deposition (IAPECVD). These plasma processing techniques often rely upon radio frequency power (rf) supplied to an inductive coil for providing power to gas phase species in forming a plasma.
Plasmas can be used to form neutral species (i.e., uncharged) for purposes of removing or forming films in the manufacture of integrated circuit devices. For instance, chemical dry etching generally depends on gas-surface reactions involving these neutral species without substantial ion bombardment.
In other manufacturing processes, ion bombardment to substrate surfaces is often undesirable. This ion bombardment, however, is known to have harmful effects on properties of material layers in devices and excessive ion bombardment flux and energy can lead to intermixing of materials in adjacent device layers, breaking down oxide and "wear out," injecting of contaminative material formed in the processing environment into substrate material layers, harmful changes in substrate morphology (e.g. amophotization), etc.
Ion assisted etching processes, however, rely upon ion bombardment to the substrate surface in defining selected films. But these ion assisted etching processes commonly have a lower selectivity relative to conventional CDE processes. Hence, CDE is often chosen when high selectivity is desired and ion bombardment to substrates are to be avoided.
One commonly used chemical dry etching technique is conventional photoresist stripping, often termed ashing or stripping. Conventional resist stripping relies upon a reaction between a neutral gas phase species and a surface material layer, typically for removal. This reaction generally forms volatile products with the surface material layer for its removal. The neutral gas phase species is formed by a plasma discharge. This plasma discharge can be sustained by a coil (e.g., helical coil, etc.) operating at a selected frequency in a conventional photoresist stripper. An example of the conventional photoresist stripper is a quarter-wave helical resonator stripper, which is described by U.S. Pat. No. 4,368,092 in the name of Steinberg et al.
Referring to the above, an objective in chemical dry etching is to reduce or even eliminate ion bombardment (or ion flux) to surfaces being processed to maintain the desired etching selectivity. In practice, however, it is often difficult to achieve using conventional techniques. These conventional techniques generally attempt to control ion flux by suppressing the amount of charged species in the plasma source reaching the process chamber. A variety of techniques for suppressing these charged species have been proposed.
These techniques often rely upon shields, baffles, large separation distances between the plasma source and the chamber, or the like, placed between the plasma source and the process chamber. The conventional techniques generally attempt to directly suppress charge density downstream of the plasma source by interfering with convective and diffusive transport of charged species. They tend to promote recombination of charged species by either increasing the surface area (e.g., baffles, etc.) relative to volume, or increasing flow time, which relates to increasing the distance between the plasma source and the process chamber.
These baffles, however, cause loss of desirable neutral etchant species as well. The baffles, shields, and alike, also are often cumbersome. Baffles, shields, or the large separation distances also cause undesirable recombinative loss of active species and sometimes cause radio frequency power loss and other problems. These baffles and shields also are a potential source of particulate contamination, which is often damaging to integrated circuits.
Baffles, shields, spatial separation, and alike, when used alone also are often insufficient to substantially prevent unwanted parasitic plasma currents. These plasma currents are generated between the wafer and the plasma source, or between the plasma source and walls of the chamber. It is commonly known that when initial charged species levels are present in an electrical field, the charged species are accelerated and dissociative collisions with neutral particles can multiply the concentration of charge to higher levels. If sufficient "seed" levels of charge and rf potentials are present, the parasitic plasma in the vicinity of the process wafer can reach harmful charge density levels. In some cases, these charge densities may be similar to or even greater than plasma density within the source plasma region, thereby causing even more ion flux to the substrate.
Charge densities also create a voltage difference between the plasma source and processing chamber or substrate support, which can have an additional deleterious effect. This voltage difference enhances electric fields that can accelerate extraction of charge from the plasma source. Hence, their presence often induces increased levels of charge to be irregularly transported from the plasma source to process substrates, thereby causing non-uniform ion assisted etching.
Conventional ion assisted plasma etching, however, often requires control and maintenance of ion flux intensity and uniformity within selected process limits and within selected process energy ranges. Control and maintenance of ion flux intensity and uniformity are often difficult to achieve using conventional techniques. For instance, capacitive coupling between high voltage selections of the coil and the plasma discharge often cause high and uncontrollable plasma potentials relative to ground. It is generally understood that voltage difference between the plasma and ground can cause damaging high energy ion bombardment of articles being processed by the plasma, as illustrated by U.S. Pat. No. 5,234,529 in the name of Johnson. It is further often understood that rf component of the plasma potential varies in time since it is derived from a coupling to time varying rf excitation. Hence, the energy of charged particles from plasma in conventional inductive sources is spread over a relatively wide range of energies, which undesirably tends to introduce uncontrolled variations in the processing of articles by the plasma.
The voltage difference between the region just outside of a plasma source and the processing chamber can be modified by introducing internal conductive shields or electrode elements into the processing apparatus downstream of the source. When the plasma potential is elevated with respect to these shield electrodes, however, there is a tendency to generate an undesirable capacitive discharge between the shield and plasma source. These electrode elements are often a source of contamination and the likelihood for contamination is even greater when there is capacitive discharge (ion bombardment from capacitive discharge is a potential source of sputtered material). Contamination is damaging to the manufacture of integrated circuit devices.
Another limitation is that the shield or electrode elements generally require small holes therein as structural elements. These small holes are designed to allow gas to flow therethrough. The small holes, however, tend to introduce unwanted pressure drops and neutral species recombination. If the holes are made larger, the plasma from the source tends to survive transport through the holes and unwanted downstream charge flux will often result. In addition, undesirable discharges to these holes in shields can, at times, produce an even more undesirable hollow cathode effect.
In conventional helical resonator designs, conductive external shields are interposed between the inductive power (e.g., coil, etc.) and walls of the vacuum vessel containing the plasma. A variety limitations with these external capacitive shielded plasma designs (e.g., helical resonator, inductive discharge, etc.) have been observed. In particular, the capacitively shielded design often produces plasmas that are difficult to tune and even ignite. Alternatively, the use of unshielded plasma sources (e.g., conventional quarter-wave resonator, conventional half-wave resonator, etc.) attain a substantial plasma potential from capacitive coupling to the coil, and hence are prone to create uncontrolled parasitic plasma currents to grounded surfaces. Accordingly, the use of either the shielded or the unshielded sources using conventional quarter and half-wave rf frequencies produce undesirable results.
In many conventional plasma sources a means of cooling is required to maintain the plasma source and substrates being treated below a maximum temperature limit. Power dissipation in the structure causes heating and thereby increases the difficulty and expense of implementing effective cooling means. Inductive currents may also be coupled from the excitation coil into internal or capacitive shields and these currents are an additional source of undesirable power loss and heating. Conventional capacitive shielding in helical resonator discharges utilized a shield which was substantially split along the long axis of the resonator to lessen eddy current loss. However, such a shield substantially perturbs the resonator characteristics owing to unwanted capacitive coupling and current which flows from the coil to the shield. Since there are no general design equations, nor are properties currently known for resonators which are "loaded" with a shield along the axis, sources using this design must be sized and made to work by trial and error.
In inductive discharges, it is highly desirable to be able to substantially control the plasma potential relative to ground potential, independent of input power, pressure, gas composition and other variables. In many cases, it is desired to have the plasma potential be substantially at ground potential (at least offset from ground potential by an amount insignificantly different from the floating potential or intrinsic DC plasma potential). For example, when a plasma source is utilized to generate neutral species to be transported downstream of the source for use in ashing resist on a semiconductor device substrate (a wafer or flat panel electronic display), the concentration and potential of charged plasma species in the reaction zone are desirably reduced to avoid charging damage from electron or ionic current from the plasma to the device. When there is a substantial potential difference between plasma in the source and grounded surfaces beyond the source, there is a tendency for unwanted parasitic plasma discharges to form outside of the source region.
Another undesirable effect of potential difference is the acceleration of ions toward grounded surfaces and subsequent impact of the energetic ions with surfaces. High energy ion bombardment may cause lattice damage to the device substrate being processed and may cause the chamber wall or other chamber materials to sputter and contaminate device wafers. In other plasma processing procedures, however, some ion bombardment may be necessary or desirable, as is the case particularly for anisotropic ion-induced plasma etching procedures (for a discussion of ion-enhanced plasma etching mechanisms See Flamm (Ch. 2, pp.94-183 in Plasma Etching, An Introduction, D. M. Manos and D. L. Flamm, eds., Academic Press, 1989)). Consequently, uncontrolled potential differences, such as that caused by "stray" capacitive coupling from the coil of an inductive plasma source to the plasma, are undesirable.
Referring to the above limitations, conventional plasma sources also have disadvantages when used in conventional plasma enhanced CVD techniques. These techniques commonly form a reaction of a gas composition in a plasma discharge. One conventional plasma enhanced technique relies upon ions aiding in rearranging and stabilizing the film, provided the bombardment from the plasma is not sufficiently energetic to damage the underlying substrate or the growing film. Conventional resonators and other types of inductive discharges often produce parasitic plasma currents from capacitive coupling, which often detrimentally influences film quality, e.g., an inferior film, etc. These parasitic plasma currents are often uncontrollable, and highly undesirable. These plasma sources also have disadvantages in other plasma processing techniques such as ion-assisted etching, and others. Of course, the particular disadvantage will often depend upon the application.
To clarify certain concepts used in this application, it will be convenient to introduce these definitions.
Ground (or ground potential): These terms are defined as a reference potential which is generally taken as the potential of a highly conductive shield or other highly conductive surface which surrounds the plasma source. To be a true ground shield in the sense of this definition, the RF conductance at the operating frequency is often substantially high so that potential differences generated by current within the shield are of negligible magnitude compared to potentials intentionally applied to the various structures and elements of the plasma source or substrate support assembly. However, some realizations of plasma sources do not incorporate a shield or surface with adequate electrical susceptance to meet this definition. In implementations where there is a surrounding conductive surface that is somewhat similar to a ground shield or ground plane, the ground potential is taken to be the fictitious potential which the imperfect grounded surface would have equilibrated to if it had zero high frequency impedance. In designs where there is no physical surface which is adequately configured or which does not have insufficient susceptance to act as a "ground" according to the above definition, ground potential is the potential of a fictitious surface which is equi-potential with the shield or "ground" conductor of an unbalanced transmission line connection to the plasma source at its RF feed point. In designs where the plasma source is connected to an RF generator with a balanced transmission line RF feed, "ground" potential is the average of the driven feed line potentials at the point where the feed lines are coupled to the plasma source.
Inductively Coupled Power: This term is defined as power transferred to the plasma substantially by means of a time-varying magnetic flux which is induced within the volume containing the plasma source. A time-varying magnetic flux induces an electromotive force in accord with Maxwell's equations. This electromotive force induces motion by electrons and other charged particles in the plasma and thereby imparts energy to these particles.
RF inductive power source and bias power supply: In most conventional inductive plasma source reactors, power is supplied to an inductive coupling element (the inductive coupling element is often a multi-turn coil which abuts a dielectric wall containing a gas where the plasma is ignited at low pressure) by an rf power generator.
Conventional Helical Resonator: Conventional helical resonator can be defined as plasma applicators. These plasma applicators have been designed and operated in multiple configurations, which were described in, for example, U.S. Pat. No. 4,918,031 in the names of Flamm et al., U.S. Pat. No. 4,368,092 in the name of Steinberg et al., U.S. Pat. No. 5,304,282 in the name of Flamm, U.S. Pat. No. 5,234,529 in the name of Johnson, U.S. Pat. No. 5,431,968 in the name of Miller, and others. In these configurations, one end of the helical resonator applicator coil has been grounded to its outer shield. In one conventional configuration, a quarter wavelength helical resonator section is employed with one end of the applicator coil grounded and the other end floating (i.e., open circuited). A trimming capacitance is sometimes connected between the grounded outer shield and the coil to "fine tune" the quarter wave structure to a desired resonant frequency that is below the native resonant frequency without added capacitance. In another conventional configuration, a half-wavelength helical resonator section was employed in which both ends of the coil were grounded. The function of grounding the one or both ends of the coil was believed to be not essential, but advantageous to "stabilize the plasma operating characteristics" and "reduce the possibility of coupling stray current to nearby objects." See U.S. Pat. No. 4,918,031.
Conventional resonators have also been constructed in other geometrical configurations. For instance, the design of helical resonators with a shield of square cross section is described in Zverev et al., IRE Transactions on Component Parts, pp. 99-110, Sept. 1961. Johnson (U.S. Pat. No. 5,234,529) teaches that one end of the cylindrical spiral coil in a conventional helical resonator may be deformed into a planar spiral above the top surface of the plasma reactor tube. U.S. Pat. No. 5,241,245 in the names of Barnes et al. teach the use of conventional helical resonators in which the spiral cylindrical coil is entirely deformed into a planar spiral arrangement with no helical coil component along the sidewalls of the plasma source (this geometry has often been referred to as a "transformer coupled plasma," termed a TCP).
From the above it is seen that an improved technique, including a method and apparatus, for plasma processing is often desired.